Make Your Own Potassium Carbonate

Tomatoes in aquaponics

There are quite a few varieties of retail products for adding Potassium to your aquaponics.  Generally Potassium hydroxide, Potassium bicarbonate and Potassium carbonate is used to buffer the pH (add alkalinity to the water) to offset the acid produced by nitrifying bacteria.   Potassium is especially helpful for those looking to get more yield or more fruit from their eggplants, tomatoes, cucumbers, peppers and other fruiting plants.

With the colder season on us in the Southern hemisphere, if you are like me you might be using a fire to heat the house (or your fish) or at least thinking about it.  This is a great time to stock up on Potassium Carbonate ready for use when Spring is planting is getting close.  Gardening is much like ants, in Winter plan for Summer and in Summer plan for Winter.  With these fires keeping us warm, we have a source of carbonates at our disposal, so why not use them in your garden?

You can simple add the ash from your fire to your garden but in an recirculating aquatic type garden like aquaponics, you will be adding quite a few things that are not necessarily useful and at times will accumulate to toxic levels for you plants.  Like Sodium for example.  So we are going to separate those things from our ash before we use it.  First we need a little understanding of how we separate out the stuff we don’t want.

Separating out the impurities we don’t want is all about how well something dissolves in water (solubility).  If you have ever dropped calcium carbonate into water and stirred it, you will have seen that most of it will not dissolve that well in water because calcium carbonate is not very soluble, hence why most will use calcium hydroxide to buffer our water instead.   The solubility of a substance is also related to heat.  Different “stuff” dissolves at different temperatures so we will use that info to separate the unwanted “stuff” from our DIY Potassium Carbonate.

Solubility of stuffMost of what is in the above table will be in our burned plant material (firewood – Oak is a good one) to some degree or another along with other “stuff” that will either float or sink out of our solution.  You can see that Potassium carbonate is highly soluble in water even when it is cold in comparison to all of the other “stuff”, but we want to get more of the Potassium carbonate out and use temperature to do that and to separate the others from the mix.  When boiled most of the “stuff” is twice as soluble which is handy to know.

DIY Potassium Carbonate

We want to make up about 5 litres of Potassium carbonate.  It wont be pure because we are doing this roughly and not filtering the water through each process and just using heat and cooling to do the job for us. So we will not be making the powder, that takes too much energy and mucking about considering we are only going to add it to water anyway.  So we will make a wet solution ready for use.

  1. We collect our ash from the fire place or you can burn wood or plants just for the experiment. Up to you.  You will want about 5kg of the ash, which is quite a bit.
  2. Put that ash into a 20 litre bucket of water.  You are best to use rain water because the tap water will have all sorts of other “stuff” to play with.  Use your fish water if you must.
  3. Give the bucket a really good stir a few times during the day and then let it settle overnight.
  4. In the morning you will find charcoal floating on the top and a mixture of the calcium carbonate (not very soluble) and dirt, silica and other insoluble “stuff” that will be of no use to you.
  5. Skim off the floating charcoal and gently decant or syphon off the clear water making sure to not disturb the “stuff” on the bottom.  Now you have removed most of the calcium carbonate, silica and other non wanted “stuff”.  Have a taste, you have nearly made soap… ewww.
  6. Now we need to get rid of the chlorides.  Because the carbonates left are more soluable than the chlorides at higher temperatures, we need to use heat.  This may mean another fire to start your new collection of ash or you could heat it on the stove.  You will want to boil about 5 litres at a time or make smaller batches.  So put your clear water in a pot and boil it.  Put a spoon or something in the pot to stop it overheating….  continue boiling it until half of the water is gone.
  7. The most of the calcium, potassium and sodium chlorides will have precipitated out leaving you with mostly sodium and potassium carbonates.   Decant (carefully) the hot water into another heat proof container and leave the solids behind.  Flush what is left behind in the pot down the drain….
  8. We don’t want the sodium carbonate so we reverse the game and cool the solution.  As it cools, because the potassium carbonate is more soluable in cool water than sodium carbonate (see the table above), the sodium will precipitate out as the solution cools.
  9. Once cool decant what is left into your storage containers and leave behind the solids again.  There wont be much but most of the solids will be sodium of one form or another and has no place in your aquaponic garden.

Now you have a solution of Potassium carbonate to use at your will in your aquaponic garden.  Test the pH of your solution.  In pure form Potassium is a base or alkaline and has a pH of 11.5 so when you test your new solution it should be quite high which will let you know you have succeeded in making your own Potassium Carbonate.  Yay!

Yes it is easier to buy the stuff but more fun to make it!

Regards
Paul

There are quite a few varieties of retail products for adding Potassium to your aquaponics.  Generally Potassium hydroxide, Potassium bicarbonate and Potassium carbonate is used to buffer the pH (add alkalinity to the water) to offset the acid produced by nitrifying bacteria.   Potassium is especially helpful for those looking to get more yield or more fruit from their eggplants, tomatoes, cucumbers, peppers and other fruiting plants.

With the colder season on us in the Southern hemisphere, if you are like me you might be using a fire to heat the house (or your fish) or at least thinking about it.  This is a great time to stock up on Potassium Carbonate ready for use when Spring is planting is getting close.  Gardening is much like ants, in Winter plan for Summer and in Summer plan for Winter.  With these fires keeping us warm, we have a source of carbonates at our disposal, so why not use them in your garden?

You can simple add the ash from your fire to your garden but in an recirculating aquatic type garden like aquaponics, you will be adding quite a few things that are not necessarily useful and at times will accumulate to toxic levels for you plants.  Like Sodium for example.  So we are going to separate those things from our ash before we use it.  First we need a little understanding of how we separate out the stuff we don’t want.

Separating out the impurities we don’t want is all about how well something dissolves in water (solubility).  If you have ever dropped calcium carbonate into water and stirred it, you will have seen that most of it will not dissolve that well in water because calcium carbonate is not very soluble, hence why most will use calcium hydroxide to buffer our water instead.   The solubility of a substance is also related to heat.  Different “stuff” dissolves at different temperatures so we will use that info to separate the unwanted “stuff” from our DIY Potassium Carbonate.

Solubility of stuffMost of what is in the above table will be in our burned plant material (firewood – Oak is a good one) to some degree or another along with other “stuff” that will either float or sink out of our solution.  You can see that Potassium carbonate is highly soluble in water even when it is cold in comparison to all of the other “stuff”, but we want to get more of the Potassium carbonate out and use temperature to do that and to separate the others from the mix.  When boiled most of the “stuff” is twice as soluble which is handy to know.

DIY Potassium Carbonate

We want to make up about 5 litres of Potassium carbonate.  It wont be pure because we are doing this roughly and not filtering the water through each process and just using heat and cooling to do the job for us. So we will not be making the powder, that takes too much energy and mucking about considering we are only going to add it to water anyway.  So we will make a wet solution ready for use.

  1. We collect our ash from the fire place or you can burn wood or plants just for the experiment. Up to you.  You will want about 5kg of the ash, which is quite a bit.
  2. Put that ash into a 20 litre bucket of water.  You are best to use rain water because the tap water will have all sorts of other “stuff” to play with.  Use your fish water if you must.
  3. Give the bucket a really good stir a few times during the day and then let it settle overnight.
  4. In the morning you will find charcoal floating on the top and a mixture of the calcium carbonate (not very soluble) and dirt, silica and other insoluble “stuff” that will be of no use to you.
  5. Skim off the floating charcoal and gently decant or syphon off the clear water making sure to not disturb the “stuff” on the bottom.  Now you have removed most of the calcium carbonate, silica and other non wanted “stuff”.  Have a taste, you have nearly made soap… ewww.
  6. Now we need to get rid of the chlorides.  Because the carbonates left are more soluable than the chlorides at higher temperatures, we need to use heat.  This may mean another fire to start your new collection of ash or you could heat it on the stove.  You will want to boil about 5 litres at a time or make smaller batches.  So put your clear water in a pot and boil it.  Put a spoon or something in the pot to stop it overheating….  continue boiling it until half of the water is gone.
  7. The most of the calcium, potassium and sodium chlorides will have precipitated out leaving you with mostly sodium and potassium carbonates.   Decant (carefully) the hot water into another heat proof container and leave the solids behind.  Flush what is left behind in the pot down the drain….
  8. We don’t want the sodium carbonate so we reverse the game and cool the solution.  As it cools, because the potassium carbonate is more soluable in cool water than sodium carbonate (see the table above), the sodium will precipitate out as the solution cools.
  9. Once cool decant what is left into your storage containers and leave behind the solids again.  There wont be much but most of the solids will be sodium of one form or another and has no place in your aquaponic garden.

Now you have a solution of Potassium carbonate to use at your will in your aquaponic garden.  Test the pH of your solution.  In pure form Potassium is a base or alkaline and has a pH of 11.5 so when you test your new solution it should be quite high which will let you know you have succeeded in making your own Potassium Carbonate.  Yay!

Yes it is easier to buy the stuff but more fun to make it!

Regards
Paul

About the author

Paul Van der Werf

Paul is the Operations Manager for a 4400m2 integrated aquaculture pilot project in the United Arab Emirates desert he designed and built. This is a commercial aquaponics pilot to evaluate integrated farming in arid climates.

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14 Comments on “Make Your Own Potassium Carbonate

  1. Chris

    Thanks Paul , excellent article again … Looking forward to making some …

    Reply
  2. Rob

    Thanks for that Paul… “Toasting marshmallows for the fish”, sounds like a good excuse to fire up the brazier to me :D
    Cheers Paul..

    Reply
  3. Jonathan

    great article.. I learnt a few interesting things from this, thank you

    Reply
  4. QwkDrw

    Aquaculture lessons. Horticulture lessons. Now we get chemistry lessons. Wow! What a deal! Are there any rocket science lessons in the future? LOL.

    Great tip Paul. You make is sound so easy. Thanks for your dedication.

    Have a great day!
    QwkDrw

    Reply
    1. lol I will do some rocket science another day. Perhaps if you need to grow on the Moon we can work on that ….

      Reply
  5. AhdRaid

    Can I use this solution to spray on Aphids, instead of liquid soap?

    thanks

    Reply
    1. Hi AhdRaid,

      I think the pH would be too high for use as a foliar spray. Never tried it though.

      Reply
  6. AhdRaid

    Thanks Paul,
    I have another question,
    With the solution that I have made, I figured out the amount of liquid needed to buffer PH of my Fish tank by 0.2 is 60ml. But the trouble is, no matter how high I set the bar, when I check after 24 hours, ph jumps back to nearly where it was. So, I guess this thing isn’t very stable. I followed all your step in making the solution.
    On the other hand, I had 4 cucumber plants and there were fruits everywhere, and they weighed more than previous batches. I guess it worked that way, for boosting fruits.
    I’m wondering how much of the solution to use per week or a particular time frame? If I added too much will there be negative effects?
    My Fish tank is 1200L connected to some gravel grow beds.

    thanks

    Reply
    1. Hi AhdRaid,

      I assume your pH jumps back down to where it was, not up?

      This is an interesting chemistry reaction in aquatic systems. When you add a carbonate, it locks out CO2 for about 24 hours, then releases it again. This gives you a false reading within that time frame because the CO2 is not converting to carbonic acid, so your pH will jump up, then when the CO2 is released again, the pH falls back down.

      It could be an indication your Alkalinity is too low. While this will happen regardless of the Alkalinity levels, when the pH falls again it should be higher than it was the next day if you have enough Alkalinity to counter the sudden increase in carbonic acid.

      Have you tested your Alkalinity?

      Cheers
      Paul

      Reply
  7. AhdRaid

    Thanks Paul for your explanation why ph jumps back.
    Yes, ph jumpsz back down. It was at about 6.4.
    I have not tested Alkalinity, cos I have no test kit at the moment. Is it very important to get one?
    I use melted coral to buffer ph up. It is very stable. 1 teaspoon of it adjusts 0.2+ ph.
    So, is there a measurement of how much potassium to use?

    Reply
    1. No worries.

      Yes alkalinity testing and management is far better than managing pH.

      You would need to know the level of carbonates in the solution you made up.

      Cheers
      Paul

      Reply
  8. AhdRaid

    Thanks Paul,
    It seems I have to get KH test kit.
    New things learned in AP :)

    thanks

    Reply
  9. sounds cool
    i actually need this KCO3 to make KOH
    I needed KOH to make KNO3
    i hope more people see this

    Reply

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